The specific monitoring of the overall state of a generator plays a large part in dealing with generators. In order to prevent idle time due to defective components, or to have a basis available for decisions about the replacement or partial replacement of generators, generators therefore have to be periodically investigated in many aspects. Downtime due to such stocktaking is to be as short as possible; that is, the inspections are to be easily possible without great dismantling work and without lengthy investigations and expense of data collection.
In particular, the investigation of the so-called machine air gap, the interspace between the stationary stator and the rotor which rotates during operation, is important here, and should be possible, preferably without expensive removal of the rotor from the stator.
Investigation of the machine air gap involves, on the one hand, obtaining information by visual methods concerning short-circuited coils in the rotor winding, the ends of the laminations, the state of the portions visible in the stator bore, such as, e.g., keys and laminations, rotor surface including keying and caps, etc.
On the other hand, other measuring processes are carried out to determine the state of the components bordering on the air gap, centered on the following: stator keying, stator lamination bundle, and lamination short circuits. The method of evaluation of the state of the stator keying is based, for example, on a system which hammer tests the wedges with a fixed force and registers the resulting vibrations. The state of the stator lamination bundle is determined by a method based on the measurement of the groove leakage flux, which varies in the presence of short circuits. The groove leakage flux is produced by ring magnetization (low induction) of the lamination bundle. Possible short circuits are thus localized by recording the leakage flux picture of the lamination edges.
According to the prior art, the investigation of the machine air gap with the rotor built in mostly takes place in that devices are attached to the fan seated on the rotor, to the stator housing, or to the stator, and make possible the introduction of inspection probes into the gap in a controlled manner. Because of different geometry of fans, stators and stator housings in different generators, large adjustments of the device to the circumstances in the generator are frequently carried out when such devices are built in.